This invention relates to optical time division communication systems and concerns in particular a method of and a device for polarization-independent optical demultiplexing of the signals transmitted in such a system, for instance, a system in which the signals are transmitted by using the Optical Time Division Multiplexing (OTD) technique.
In an OTD system, the optical pulses corresponding to the different tributary channels are interleaved into a single stream, in different time positions. In this way very high bit rate transmissions (even in the order on hundreds of Gbits/s) are obtained without the need for using particularly sophisticated opto-electronic components for the transmission and reception of the individual tributaries. At such bit rates, demultiplexing must be effected in a fully optical way and one of the commonly used techniques for this purpose is the so-called four wave mixing. The phenomenon of four wave mixing is a non-linear phenomenon wherein an information signal at a frequency fs of sufficiently high power, is made to interact with a pair of pump or control signals, having respective frequencies fp1, fp2 different from fs and also having sufficiently high power, and a mixing signal at a frequency fp1, +fp2xe2x88x92fs is generated.
Very often use is made of only one control signal at a frequency fp, thus obtaining a mixing signal at a frequency 2fpxe2x88x92fs (partially degenerate four wave mixing).
For the optical demultiplexing of an OTD stream through four wave mixing, the multiplexed stream and control pulses, which have the same repetition frequency as the tributaries and are time-aligned with the pulses of the channel to be extracted, are fed into a non-linear optical medium, and at the output a filtering is effected to maintain the mixing signal only. An example of this technique is described in the paper xe2x80x9c16 Gbit/s all optical demultiplexing using four wave mixingxe2x80x9d by P. A. Andrekson et al., Electronics Letters, Vo. 27, No 11, May 23, 1991, pp. 922 to 924.
This demultiplexing technique is particularly simple, since it does not use interferometric structures, and it is efficient also at very high bit rates; nevertheless its efficiency is strongly dependent on the state of polarization of both the information signal and the control signals. Now, considering that the optical fiber transmission lines commonly used in telecommunications are not made up of polarization maintaining fibers, the need arises of ensuring in the demultiplexer the correct relation between the state of polarization of both the control signals and the signals to be demultiplexed.
Solutions based on polarization diversity schemes meeting this requirement have been already proposed in the literature.
The paper xe2x80x9cPolarization-independent 100 Gbit/s all optical demultiplexing using four-wave mixing in a polarization maintaining fibre loopxe2x80x9d by T.Morioka et al., Electronics Letters, Vol. 30, No. 7, Mar. 31, 1994, pp 591-592, discloses effecting the four wave mixing in a loop consisting of a dispersion shifted and polarization maintaining fiber, in which the two outputs with orthogonal polarizations of a polarizing beam splitter are aligned with a same eigenaxis of the polarization maintaining fibre. Since the mixing efficiency per length unit is very low in dispersion shifted and polarization-maintaining fibers, it is necessary to use a rather long fiber section (of the order of some kilometers). The dispersion shifted and polarization maintaining fibers are rather expensive and this makes using such devices within the network particularly costly, considering that a demultiplexer for each channel is needed.
A second known solution is described in the article xe2x80x9cPolarization-independent alloptical demultiplexing up to 200 Gbit/s using four-wave mixing in a semiconductor laser amplifierxe2x80x9d by T.Morioka et al, Electronics Letters, Vol. 32, No 9, Apr. 25, 1996, pp 840 to 842. The article describes a demultiplexing method wherein the four wave mixing is performed in an optical semiconductor amplifier, connected between two identical sections of a polarization maintaining fiber, so arranged that the birefringence axes of one of the sections are rotated by 90xc2x0 with respect to those of the other. In this way the depolarization effect of the first fiberxe2x80x94which results in a time separation of the pulses propagating along the fast axis and the slow axisxe2x80x94is compensated by the second fiber. However, the compensation takes actually place only if the two fiber sections have exactly the same length and are perfectly oriented. Both conditions are rather difficult to achieve in practice, in particular at the time of the industrial production of a component.
According to this invention, a method and a device are instead provided that are self-aligning and therefore do not require any critical mounting operations or use of long sections of special and expensive fiber.
In particular, the invention concerns a method wherein the signals of the stream to be demultiplexed and the control signals are fed into a section of polarization maintaining fiber that separates said signals into two components with orthogonal polarization and separated in time, and the four wave mixing is effected separately on said two components, thereby originating two individual mixing signals. According to the invention two components are fed into a dispersion shifted fiber section, in which said separate mixing takes place, and the individual mixing signals are subjected to a reflection with a 900 rotation of the state of polarization and are sent towards a receiver, passing again through the dispersion shifted fiber section and the polarization maintaining fiber section, where joining between said mixing signals occurs.
The invention also concerns the device for implementing the method. Said device comprises: means for bringing the stream to be demultiplexed and control signals, that occur at a rate equal to the rate of the pulses of a tributary channel and are time-aligned with the pulses of a channel to be extracted from the stream, to such a power level as to allow originating non-linearity phenomena; means, including a polarization maintaining fiber section, for splitting the optical signals of said stream and the control signals into two time-separated components with orthogonal state of polarization; means for effecting a four wave mixing between the signals relating to the channel to be extracted and the control signals, separately for said two components; filtering means for separating the mixing signals relating to the two components; and means for combining into a single signal the mixing signals relating to the two components.
According to the invention, the mixing means comprise a dispersion-shifted fiber section that is connected to the polarization maintaining fibre section, conveys the mixing signals relating to the two polarization components towards means arranged to reflect such mixing signals and to rotate their polarization by 90xc2x0, and collects and sends the reflected signals into the same polarization maintaining fiber section that had caused the separation between the components, so that such section constitutes also the means of combining into a single signal the mixing signals relating to the two components.